The present invention relates to a focus error detecting apparatus suitable for use in a compact disk player, a video disk player, other optical disk players, and so on.
According to the present invention, a light detector is divided substantially into three portions and a central one of the three portions is formed to be substantially longer than the portions on both sides thereof.
When a signal is recorded onto or reproduced from a given media such as an optical disk, it is necessary to focus light on the media.
FIG. 12 is a block diagram illustrating a conventional focus error detecting apparatus for controlling the focused condition and FIG. 13 is a circuit diagram for conducting an automatically focusing operation on the basis of a reflected laser beam from the media.
In FIG. 12, a light beam emitted from the laser source 10 is passed through a beam splitter 11 and an objective lens 12, and then focused on an information disk 13. A reflected light beam having information recorded on the disk is directed through the objective lens 12, the beam splitter 11 to a cylindrical lens 14 to make an astigmatic aberration of the reflected laser beam. The longitudinal magnification and the lateral magnification of the cylindrical lens 14 are different due to its curved configuration, so that the light transmitted through the cylindrical lens 14 forms different projected images as different positions. As shown in FIG. 12, the cylindrical lens 14 is disposed with the longitudinal axis along the x-axis. When the light beam is projected into the cylindrical lens, a narrow projected image 16 oriented in the x-axis direction is formed on a plane near the cylindrical lens 14. A narrow projected image 17 oriented in the y-axis direction is formed on a plane distanced from the cylindrical lens 14. Further, a substantially circular projected image 18 is formed on a plane between the above two planes. A light detector 1 as shown in FIG. 13 is disposed in the passage of the light beam from the cylindrical lens 14. The light detector 1 is divided substantially into three portions A, B and C. The portions A and C are arranged to either side of the portion B. The respective outputs of the portions A and C are added by an adder 3. The respective outputs of the portion B and the adder 3 are received at two inputs of a differential amplifier 2 which outputs the difference between the inputs. The output F of the differential amplifier 2 is expressed by F=.beta.B-(.alpha.A+.gamma.C), and this output F constitutes the focus error signal (here .alpha., .beta. and .gamma. are constants, and hereinafter it is assumed that each of .alpha., .beta. and .gamma. is equal to 1 for the sake of simplification). The output of the differential amplifier 2 is applied to a lens driving circuit 19 to move the objective lens 12 toward or away from the information disk 13.
When the light beam transmitted through the objective lens 12 is focused to form accurately a convergence point on the information disk 13, a circular projected image indicated by a solid line (20) is formed on the light detector 1 and the output of the amplifier 2 becomes zero, so that the lens driving circuit 19 instructs the objective lens 12 to remain fixed at the present position. When the objective lens 12 is much close to the information disk 13 and the projective image as indicated by a dotted line (22) is formed on the light detector 1, the amount of the light incident upon the portion B is smaller than the total amount of the light incident upon both portions A and C. The added signal which is the sum of the signals detected by the portions A and C is larger than a signal detected by the portion B, and therefore the differential amplifier 2 outputs a positive signal to the lens driving circuit to thereby move the objective lens 12 apart from the information disk. When the objective lens 12 is much distant from the information disk 13 and the projected image as indicated by a dotted line (21) is formed on the light detector 1, the amount of the light incident upon the portion B is larger than the total amount of the light incident upon the portions A and C. Accordingly, the added signal which is the sum of the signals detected by the portions A and C is smaller than the signal detected by the portion B, and the amplifier 2 outputs a negative signal to the lens driving circuit 19 to thereby move the objective lens toward the information disk 13.
FIG. 14 shows an enlarged view of the light detector as shown in FIG. 13 and various projected images thereon. The light spot on the light detector 1 is adjusted so as to be substantially circular in a focused condition as shown in FIG. 14(c). If the focus is shifted, in one direction, out of the focused condition, the light spot becomes a focal line as shown in FIG. 14(b), and, if the focus is shifted further in the same direction, the light spot becomes an approximate ellipse which is elongated longitudinally (in the direction of the focal line) as shown in FIG. 14(a).
If the focus is reversely shifted, the light spot becomes a focal line, as shown in FIG. 14(d), extending in the direction substantially perpendicular to the focal line shown in FIG. 14(b), and if the focus is further shifted in the same direction, the form of an approximate ellipse which is elongated transversely (in the direction of the focal line) becomes larger and larger as shown in FIGS. 14(e) and (f).
Assume now that the length of each of the portions A, B and C is selected to be 500 .mu.m (in the direction of the division lines that is, in the horizontal direction in the drawings), that the width of each of the portions A and C is selected to be 130 .mu.m, that the width of the portion B is selected to be 80 .mu.m, that the diameter of the circular light-spot in the focused state is set to be 100 .mu.m, that the distance between the two focal lines is set to be 200 .mu.m, and that the energy distribution in the light spot is uniform for the sake of simplification. Then, the sum of the respective outputs of the portions A and C of the light detector 1 (that is, the output of the adder 3) and the output of the portion B of the light detector 1 change as shown in FIG. 15 correspondingly to the focusing condition. As the result, the focus error signal F(=B-(A+C)) changes as shown in FIG. 16. Reference characters a to f in FIG. 16 correspond to reference characters a to f in FIG. 14.
As will be apparent from FIG. 16, the level of the signal F becomes zero at the position c, and the signal F is reversed in polarity between the positions b and d. Accordingly, it is possible to control the focus error using the signal F.
The signal F, however, has another zero-crossing point at the position e. Therefore, when the focus servo is turned on, it is necessary to distinguish the position c from the position e, for example, on the basis of the signal level, so as to prevent the servo from being locked-in at position If this point is disregarded, the pull-in operation becomes unstable.